Young Folks' Library, Volume XI (of 20) eBook

upon the clouds sufficiently distinct, and for a sufficiently
long time, to permit of my taking a sketch in my journal
and studying the physical condition of the clouds upon
which it was produced. I was able to determine
directly the circumstances of its production.
Indeed, as this brilliant phenomenon occurred in the
midst of the very clouds which I was traversing, it
was easy for me to ascertain that these clouds were
not formed of frozen particles. The thermometer
marked 2 deg. above zero. The hygrometer marked
a maximum of humidity experienced, namely, seventy-seven
at three thousand seven hundred and seventy feet,
and the balloon was then at four thousand six hundred
feet, where the humidity was only seventy-three.
It is therefore certain that this is a phenomenon
of the diffraction of light simply produced by the
vesicles of the mist.

The name of diffraction is given to all the modifications
which the luminous rays undergo when they come in
contact with the surface of bodies. Light, under
these circumstances, is subject to a sort of deviation,
at the same time becoming decomposed, whence result
those curious appearances in the shadows of objects
which were observed for the first time by Grimaldi
and Newton.

The most interesting phenomena of diffraction are
those presented by gratings, as are technically
denominated the systems of linear and very narrow
openings situated parallel to one another and at very
small intervals. A system of this kind may be
realized by tracing with a diamond, for instance,
on a pane of glass equidistant lines very close together.
As the light would be able to pass in the interstices
between the strokes, whereas it would be stopped in
the points corresponding to those where the glass
was not smooth, there is, in reality, an effect produced
as if there were a series of openings very near to
each other. A hundred strokes, about 1/25th of
an inch in length, may thus be drawn without difficulty.
The light is then decomposed in spectra, each overlapping
the other. It is a phenomenon of this kind which
is seen when we look into the light with the eye half
closed; the eyelashes in this case, acting as a net-work
or grating. These net-works may also be produced
by reflection, and it is to this circumstance that
are due the brilliant colors observed when a pencil
of luminous rays is reflected on a metallic surface
regularly striated.

To the phenomena of gratings must be attributed, too,
the colors, often so brilliant, to be seen in mother-of-pearl.
This substance is of a laminated structure; so much
so, that in carving it the different folds are often
cut in such a way as to form a regular net-work upon
the surface. It is, again, to a phenomenon of
this sort that are due the rainbow hues seen in the
feathers of certain birds, and sometimes in spiders’
webs. The latter, although very fine, are not
simple, for they are composed of a large number of
pieces joined together by a viscous substance, and
thus constitute a kind of net-work.